I. Field of the Invention
The present invention relates to a tilt sensor for detecting the presence/absence of a tilt or an acceleration of an object when the object is tilted or vibrated at a lateral acceleration.
II. Description of the Prior Art
According to a proposed tilt sensor, a permanent magnet having a surface coated with a magnetic fluid is housed in a hollow case, and a magnetic sensing element is arranged outside the case (U.S. Ser. No. 112,626 and E.P.C. application Ser. No. 87115618.8). With this arrangement, the permanent magnet is moved to a lower position by its own weight when the case is tilted. As a result, magnetism detected by the magnetic sensing element is changed, and hence the tilt sensor detects that the case is tilted.
FIG. 15 shows a tilt sensor of this type. Reference numeral 1 denotes a nonmagnetic case made of aluminum. A cup-like case main body 1a and a cover 1b of the case 1 are bonded to each other by an adhesive applied between overlapped portions thereof, so that the case 1 has a sealed hollow portion 2. A substantially disk-like permanent magnet 4 having a surface coated with a magnetic fluid 3 is housed in the hollow portion 2. Note that reference numeral 1c denotes a small hole through which the magnetic fluid 3 is injected into the case 1 after the permanent magnet 4 is placed in the case 1 and the case main body 1a and the cover 1b are bonded to each other by the adhesive. After the injection of the magnetic fluid 3, the small hole 1c is sealed. Reference numeral 5 denotes a sensor mounting base consisting of a plastic molded member. The case 1 is inserted in an upper portion of the mounting base 5 and is fixed thereto by a resin 6. Reference numeral 7 denotes a reed switch as a magnetic sensing element arranged on a printed board 8 and having soldered lead terminals. Leads 9 extend from the printed board 8. Reference numeral 10 denotes a lower cover fixed to the mounting base 5 by engaging pawls 10a protruding from a pair of opposite sides of the cover 10 with recesses formed in the inner wall of the mounting base 5; and 11, an amorphous magnetic piece bonded to the top portion of the mounting base 5. The response angle of the tilt sensor is adjusted by a position where the amorphous magnetic piece 11 is bonded. Reference numeral 12 denotes a label on which the name, a model number, the name of a manufacturer, and the like of a tilt sensor are displayed.
In such an arrangement, while the case 1 is not tilted, the permanent magnet 4 is located at a central portion of the case 1. Since a pair of lead terminals of the reed switch 7 are equally influenced by the magnetism of the permanent magnet 4, they are polarized to the same pole at a central portion of the switch 7 and repulse each other. As a result, the switch 7 is set in an open state. When the case 1 is tilted, the permanent magnet 4 is moved to a lower position, and only one lead terminal is influenced by the magnet 4. As a result, the lead terminals are polarized to the opposite poles at the central portion of the switch 7, and hence the switch 7 is set in a connected state. Note that the magnetic fluid 3 serves as a lubricating oil when the permanent magnet 4 is moved. In this manner, a tilt of the case 1 can be detected by an opening/closing operation of the reed switch 7.
In the prior art having the above-described arrangement, however, the following problems are posed.
(1) As described above, the nonmagnetic case 1 is designed such that the permanent magnet 4 is placed in the cup-like case main body 1a, and the case main body 1a and the cover 1b coated with the adhesive, which is applied at the overlapped portions in advance, are engaged with each other to be bonded. Then, after the magnetic fluid 3 is injected through the small hole 1c formed in the cover 1b, the small hole 1c is sealed. An adhesive is also used at this time. The case 1 sealed in this manner is inserted from an opening of a bottom portion of the mounting base 5. The resultant structure is then turned upside down, and the resin 6 is supplied therein, so that the case 1 is fixed to the base 5 by curing the resin 6. The adhesive and resin must be thermally cured upon each operation. In addition, a two-part liquid type adhesive which is not affected by the magnetic fluid 3 is used as the adhesive. Therefore, the manufacture of the sensor requires a long total manufacturing time including a time required for preparation of the adhesive, and the yield is greatly decreased.
(2) After the case 1 is fixed to the upper portion of the mounting base 5, the printed board 8 having the reed switch 7 mounted thereon is inserted in the base 5. The printed board 8 is then positioned by a stopper (not shown) and is fixed by fitting the lower cover 10 in the base 5. A distance between the permanent magnet 4 and the reed switch 7 formed at this time greatly influences the response angle of the tilt sensor. In the arrangement shown in FIG. 15, a mounting position of the case 1 varies depending on the bonded state of the cup-like case main body 1a and the cover 1b, the sealed state of the small hole 1c, the fixed state of the case 1 by the resin 6, and the like. In addition, if the adhesive overflows and a projection is formed, a gap is formed between the cover 1b and the ceiling of the mounting base 5. Hence, the response angle tends to vary.
(3) The permanent magnet 4 housed in the case 1 must have a certain weight so as to increase the response speed with respect to a tilt. This is because as a weight W is increased, a component of force Wsin.alpha. for moving the magnet 4 along a tilted bottom surface of the case 1 when the case 1 is tilted at an angle .alpha. is increased. As a result, the component of force Wsin.alpha. can overcome various forces for blocking the movement of the magnet 4, such as an attraction between the magnet 4 and the of the reed switch 7 and the viscosity and surface tension lead terminals of the magnetic fluid 3.
In addition, the magnetic fluid 3 serves as a lubricating oil upon movement of the permanent magnet 4. More specifically, since the magnet 4 attracts the magnetic fluid 3, the fluid 3 flows into a space between the magnet 4 and the bottom surface of the case 1 and adheres thereto. This adhesive force serves as a force for flowing the permanent magnet 4. Consequently, the magnet 4 is moved in response to a tilt or vibrations of the case 1 while it is floated from the bottom surface of the case 1. Since a frictional resistance upon movement of the magnet 4 is generated only between the fluid 3 and the bottom surface of the case 1, it is very small. Although the magnetic fluid 3 is required only between the magnet 4 and the bottom surface of the case 1, the fluid 3 actually adheres to the permanent magnet 4 so as to cover its entire surface. The fluid 3 adhering to upper and side surfaces of the magnet 4 is unnecessary.
(4) When the reed switch 7 is arranged to be perpendicular to the bottom surface of the case 1, a distance between the bottom surface of the case 1 and the lead terminals of the reed switch 7 sensitively affects the sensitivity of the tilt sensor or a tilt angle at which the reed switch 7 responds. Therefore, by adjusting this distance, the sensitivity and the response angle can be adjusted. In the prior art, threads are formed in the outer circumference of a hold member for holding the reed switch and in the inner circumferential surface of the mounting base, which is brought into contact with the outer circumferential surface of the hold member, so as to be meshed with each other, and the hold member is pivoted to be vertically moved, thereby performing the adjustment.
Such a thread arrangement, however, undesirably increases the manufacturing cost.
(5) Since the tilt sensor incorporates the permanent magnet and the magnetic sensing element, if an instrument or a circuit susceptible to magnetic influences is present around the tilt sensor, it is influenced by a magnetic field generated by the tilt sensor. Furthermore, in contrast with this, if an external magnetic field is present near the tilt sensor, the magnetic sensing element may be erroneously operated. Therefore, the tilt sensor must be magnetically shielded from its surroundings.